Transmitter Gain

What is the purpose of adjusting the transmitter gain/attenuation?

This step in prescan, also known as pulse amplitude calibration, determines the RF output necessary to achieve a 90° (or other flip angle) pulse. Recall from a prior Q&A that the flip angle (α) for a hard (rectangular) RF-pulse is given by

α = γ • B1• tp

where γ is the gyromagnetic ratio and tp represents the length of time the B1 field is applied. The pulse length (tp) is typically fixed, and so the flip angle is controlled by varying the magnitude of the B1 field. The magnitude of B1, in turn, is dictated by the output voltage and current of the RF-amplifier. This amplifier output is scaled downward from its maximum value by means of an interposed circuit (the transmitattenuator). The attenuator controls the gain of the amplifier so that the proper current and voltage are sent to the coil to generate a B1 field of appropriate magnitude. The gain/attenuator setting must be adjusted for each patient empirically because variable loading of the transmitter coil alters the required amplifier output case by case.

Transmit attenuation is set automatically on all scanners although manual overrides are possible. The scanner accomplishes this task by sending out test RF pulses at variable attenuator settings and noting the amplitude of each FID signal created. When a maximum value is obtained, the attenuator is calibrated for a 90°-pulse. From this, the attenuation values required for a 180° or other flip-angle pulse can be calculated by simple scaling.

For non-rectangular pulses, the flip angle is proportional to the area under the RF-pulse rather than simply B1tp. And for adiabatic pulses, this relationship breaks down completely.

A variety of clever methods using multiple simultaneously acquired echoes have been developed to speed up the process of finding the optimal transmitter gain. See the patent literature in "References" below where several of these methods are explained.